In the processing of comminuted cellulosic fibrous material, for example, wood chips, to produce cellulose pulp, one of the somewhat essential devices used to introduce a pressurized slurry of material to a treatment vessel is what is known in the art as the High Pressure Feeder (xe2x80x9cHPFxe2x80x9d). The HPF is a rotary valve-type device that, with the aid of a high-pressure pump, transfers a slurry of material and liquid at one pressure for example, between about 0 to 2 bar gauge, to a second higher pressure, for example, between about 5-15 bar gauge, at which treatment of the material is most desired. One advantageous function of this device is the capability to act as an pressure isolation device. Should a disruption in the operation of the digester or the feed system occur, the HPF prevents the high-pressure medium from escaping to the low-pressure medium or to the surrounding environment.
Since the early development of the continuous cooking process by the late Johan Richter and others (as documented in Mr. Richter""s The History of Continuous Cooking [1981]), the HPF has been an essential feature of the feed system of the continuous digester. In this 1981 publication Mr. Richter documented the early development of the HPF, in particular some early designs are shown in FIGS. 17, 18, 21, and 22 of this publication. Rydholm also documents one early xe2x80x9cbalanced rotorxe2x80x9d HPF design in FIG. 1.1 of the 1970 publication Continuous Pulping Processes. The development of HPF design is also documented in U.S. Pat. Nos. 2,459,180; 2,688,416; 2,870,009; 2,901,149; 2,914,223; 3,041,232; 4,033,811; 4,338,049; 4,430,029; 4,508,473; and 4,516,887. Not until the recent development of the slurry-type pumping of the material by Prough, et al., as described in U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025; 5,736,006; 5,753,075; 5,766,418; and 5,795,438 and marketed under the name LO-LEVEL(copyright) Feed System by Ahstrom Machinery Inc. of Glens Falls, N.Y., has the elimination of the HPF and pumping directly to the treatment vessel been technically feasible.
However, the present design of the HPF, as exemplified by the designs shown in U.S. Pat. Nos. 5,236,285 and 5,236,286, has not progressed significantly since the earlier designs developed by Richter, et al. The recent development of digester feed system technology, as exemplified by the work performed in the development of the LO-LEVEL(copyright) Feed System, and documented in U.S. Pat. No. 5,476,572 and the other patents listed above, has resulted in new insights into the limitations of existing HPF designs and how these limitations can be overcome by improving the HPF as a result of these insights. The present invention is an example of such an improvement.
As shown in FIGS. 3, 4 and 5 of U.S. Pat. No. 5,236,285, the HPF comprises or consists of a stationary housing with a pocketed cylindrical rotor mounted for rotation in the housing. The housing includes four ports: a high-pressure inlet port; a high-pressure outlet port; a low-pressure inlet port and a low-pressure outlet port. The low-pressure inlet is opposite the low-pressure outlet and the high-pressure inlet is opposite the high-pressure outlet. As the pocketed rotor (driven by a variable speed motor and gear reducer) rotates in the housing, the through-going pockets of the rotor sequentially communicate with the four ports of the housing. Typically, the rotor contains two or more through-going pockets such that different pockets communicate with different high and low-pressure ports as the rotor rotates. The unique, hydraulically-balanced design of the HPF permits the rotor pockets to be exposed to high and low pressure fluids simultaneously without causing a load imbalance and excessive wear of the rotor or its lining.
Typically, the top port of the feeder housing of the HPF is the low-pressure inlet port into which a slurry of chips and liquid is introduced to the feeder. This historically has been true for over thirty years since the slurry of chips and liquor have been introduced to the HPF by gravity from a conduit, known in the art as the Chip Chute, mounted above the HPF. However, due to the pump-feeding which characterizes the LO-LEVEL Feed System marketed by Ahistrom Machinery Inc., the pressurized slurry flow from the slurry pump may be introduced to a low-pressure inlet of the HPF which is oriented wherever necessitated by the installation. The pump-fed slurry can be introduced to a port located physically on top, on either side, on the bottom of the HPF, or even to a port oriented at an oblique angle, that is, at any angle of orientation desired. However, for the sake of illustration, the low-pressure inlet of the HPF of the present invention will be assumed to be located on top of the feeder, for example, as shown in FIGS. 3-5 of U.S. Pat. No. 5,236,285. The rotor typically rotates at a speed of between about 5 to 15 rpm, preferably, between about 7 to 10 rpm, depending upon the capacity of the HPF and the production rate of the pulping system it is used to feed.
As the low-pressure slurry is introduced to the low-pressure inlet of the HPF, one or more of the through-going pockets of the rotating rotor receive the slurry. As noted above, the low-pressure outlet of the HPF is located opposite the low-pressure inlet. Therefore, as the slurry is introduced to the low-pressure inlet and the first end of one of the through-going pockets, the slurry flows into the pocket and toward the second end of the pocket, in this case, toward the lower end of the pocket, and toward the low-pressure outlet. The low-pressure outlet port of the HPF is typically provided with a screen element, for example, a cast horizontal bar type screen element (see for example the screen element 29 in U.S. Pat. No. 5,443,162). This screen element retains the chips in the slurry within the feeder and allows some of the liquid in the slurry to pass out of the second end of the pocket and through the screen. This liquid typically is recirculated back to a location upstream of the HPF. The chips that are introduced to the rotor pocket, including those chips retained by the screen element, are transported by the rotation of the rotor. After a typical one-quarter revolution of the rotor, the first end of the pocket that was once in communication with the low-pressure inlet is placed in communication with the high pressure outlet. The high-pressure outlet typically communicates with the inlet of a digester, either a continuous or batch digester, via one or more conduits. At the same time, the rotation of the rotor also places the second end of the through-going pocket, which was just in communication with the low-pressure outlet, in communication with the high-pressure inlet. The high pressure inlet typically receives a flow of high-pressure liquid from a high-pressure hydraulic pump. The pressure of this liquid typically ranges from about 5 to 15 bar gauge, and is typically about 7-10 bar gauge. This high-pressure liquid displaces the slurry of chips and liquid from the through-going pocket and out of the high-pressure outlet and ultimately to the inlet of the digester.
As the rotor continues to rotate, the second end of the pocket which received the high-pressure fluid then is placed in communication with the low-pressure inlet and receives another supply of slurry from the conduit connected to the low-pressure inlet. Similarly, the first end of the pocket is rotated into communication with the low-pressure outlet of the housing, having the screen element. The process described above then repeats itself such that during one complete revolution of the rotor each through-going pocket receives and discharges two charges of chips and liquid. The rotor typically contains at least two, typically four, through-going pockets such that the rotor is repeatedly receiving slurry from the low-pressure inlet and discharging slurry out the high-pressure outlet. The ends of the these pockets act as both an inlet for slurry and an outlet depending upon the orientation of the rotor.
Over the years, certain modifications have been made to the rotor or housing in order to improve the operation or efficiency of the HPF. As shown in U.S. Pat. No. 5,236,285 one such modification was made to the screen element in the low-pressure outlet in which the leading edge of the screen was blanked off. As the rotor rotates, this blanking of the screen postpones the exposure of the pocket to the suction pressure of the pump typically attached to the low-pressure outlet of the feeder. This prevents the screen from being blinded over with small wood chips, that is, fines and pins, prior to exposing the pocket to the full suction of the pump below. U.S. Pat. No. 5,236,285 also discloses a modification of the high-pressure inlet which minimizes the compression of the chips in the pocket due to the high-pressure introduced to the pocket by the high-pressure inlet. In a fashion similar to the screen modification discussed above, this modification to the leading edge of the high-pressure inlet comprises or consists of a barrier, or xe2x80x9cpre-pressurization wedgexe2x80x9d [See item 46 of FIG. 5 of U.S. ""285.], which prevents the pocket from being exposed to high-pressure liquid, which can compress the chips, prior to the pocket communicating with the high-pressure outlet. As a result, the uncompressed chip slurry is more easily discharged from the pocket and out the high-pressure outlet.
U.S. Pat. No. 5,236,286 discloses a method of improving the filling efficiency of a HPF by exposing the two sets of rotor pockets with an isolated supply of suction at the low-pressure outlet. The pump suction at the low-pressure outlet is simply isolated into two separate conduits.
U.S. Pat. No. 5,443,162 discloses a method of increasing the efficiency of the HPF by increasing the spacing between horizontal bars in the screen element while stiffening the screen assembly by placing a reinforcing bar at the mid-span of the bars. The increased bar spacing provides for more open flow area and thus less undesirable pressure drop across the screen element.
The present invention provides further improvements to the efficiency and operation of the HPF.
During development and evaluation of the LO-LEVEL Feed System, it was discovered that the discharge of slurry from the pockets of the HPF is hampered by the geometry of the pockets. Since the rotor is exposed to both high and low pressure liquids simultaneously, in conventional HPFs the potential of producing a load imbalance on the rotor, which might precipitate accelerated wear, is minimized by using a unique over-lapping pocket geometry. As a result, each pocket is not uniform in dimension but necks-down to a minimum dimension or xe2x80x9cthroat areaxe2x80x9d as the pocket passes through the rotor. This throat area defines the minimum flow area of each pocket. However, there are regions within and outside the throat area where the area of flow is restricted due to the geometry of the pocket. These restrictions or xe2x80x9cnooks and cranniesxe2x80x9d in the flow area of the pocket limit, if not restrict, the flow through the pocket. It is possible that such narrow pocket dimensions form areas where the flow of slurry stagnates and interferes with the evacuation of slurry from the pocket when the pocket is exposed to the high-pressure liquid introduced at the high-pressure inlet.
One aspect of the present invention overcomes this resistance to evacuating the pocket by providing a means for delaying the exposure of the pocket to the high-pressure liquid so that when the pocket is exposed to the high-pressure liquid an increased flow velocity through the pocket is obtained. This increased flow velocity, over a typically shorter period of time, aids in propelling the slurry out of the pocket, including out of the areas where the flow is restricted, so that the pocket is more thoroughly and completely emptied. For example, calculations indicate that without any form of restriction in the high pressure inlet, the inlet of the pocket would be exposed to the flow of high-pressure liquid for about 1.0 secs. Under current practice, using the present height of the pre-pressurization wedge this exposure time is reduced to about 0.9 secs. However, using the present invention, the exposure time is further reduced to about 0.8 secs. That is, the essentially same volume of flow is passed through the pocket volume in less time such that the rate of flow is greater. This greater flow rate can aid in the removal of slurry from the pocket, especially from the restricted flow areas of the pocket.
One form of the means for delaying the exposure of the pocket to high pressure liquid, and thus of achieving the desired increased flow velocity according to the invention, is to reduce the size of the opening in the high-pressure inlet so that it is smaller than conventional, for, example at least about 10%, preferably at least about 20%, and even as much as at least about 50% smaller, than the conventional high-pressure inlet opening. This reduction in area may be achieved, in one example, by increasing the size of the xe2x80x9cpre-pressurization wedgexe2x80x9d disclosed in U.S. ""285. For example, where the wedge used in conventional HPFs may be about 4.5 inches in height above the inside surface of the high-pressure outlet, so that the open area of the outlet is about 30% less than the largest cross-section of the outlet, for the present invention, the wedge height is one that preferably results in a reduction in outlet area of at least about 40% less, more preferably at least about 50% less than the largest open area of the outlet. For example, compared to conventional xe2x80x9cpre-pressurization wedgesxe2x80x9d, a wedge used according to the present invention is preferably about 2 inches taller and reduces the open area of outlet by at least about 40% compared to the largest area of the outlet. Other structures besides wedges can be used, however; any conventional structure that can achieve this goal being suitable.
The invention can be effected by casting the desired structure into the high-pressure inlet of a newly designed HPF, or existing HPFs may be modified by welding steel blocks into the inlet and machining the blocks to the desired dimensions. These blocks or pre-machined wedges may also be attached by any other conventional connecting device, for example, by a bolted connection.
In another and likely most significant embodiment of the present invention, the restriction in flow through the pocket is minimized by eliminating the nooks and crannies in the pocket geometry to provide a more uniform flow area. This can either be achieved by modifying an existing pocket geometry, for example, by xe2x80x9cfilling inxe2x80x9d the areas of the pocket that restrict flow or by fabricating new rotors having though-going pockets having more uniform geometry, that is, a geometry that is substantially devoid of the flow restrictions that are characteristic of the prior art HPF rotors. This may even be achieved by fabricating HPF rotors using cylindrical or polygonal pipe or tubing so that the pocket geometry becomes essentially uniform in cross section from one end of the pocket to the other. Such a fabrication has the further advantage of providing smoother flow surfaces, along the inside of the pipes or tubes, than are presently available in the present cast rotor design.
According to one aspect of the present invention there is provided a high pressure transfer device comprising: A housing. A pocketed rotor containing a plurality of through going pockets, the rotor rotatable about a given axis of rotation and the pockets having opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor and the pockets are provided in at least first and second sets. A housing enclosing the rotor, the housing having an exterior periphery and first through fourth ports disposed around the exterior periphery thereof for registry with the inlets to and outlets from the through going pockets, for each set; for each set the first port being opposite the third port; and the second port opposite the fourth port. The rotor mounted in the housing for rotation with respect to the ports about the given axis of rotation. And, the pockets having an interior surface configuration substantially devoid of nooks, crannies, and related flow restrictions.
The interior surface configuration of the pockets may be defined by substantially smooth interior wall tubes. For example the tubes may be substantially polygonal in cross-section over at least a majority of the length thereof, but also may be circular or elliptical. Also at least some of the tubes may be mounted in a substantially cruciform position within the rotor.
Alternatively the interior surface configuration comprises inserts substantially filling pre-existing nooks, crannies, and related flow restriction. The inserts may be substantially solid metal, may be initially fluid but hardenable wear resistant material such as epoxy or cement, and/or may be hollow or partially hollow metal inserts.
The first port may comprise a high pressure inlet port, and the device may further comprise a high pressure inlet port configuration having an opening adjacent the rotor at least 40% less in cross-sectional area than the largest cross-sectional area of the high pressure inlet.
According to another aspect of the present invention there is provided: a high pressure transfer device comprising: A housing. A pocketed rotor containing a plurality of through going pockets, the rotor rotatable about a given axis of rotation and the pockets having opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor and the pockets are provided in at least first and second sets (preferably with the pockets in each set offset from the pockets in the at least one other set). A housing enclosing the rotor, the housing having an exterior periphery and first through fourth ports disposed around the exterior periphery thereof for registry with the inlets to and outlets from the through going pockets, for each set; for each set the first port being opposite the third port; and the second port opposite the fourth port. The rotor mounted in the housing for rotation with respect to the ports about the given axis of rotation. And, the pockets defined by substantially smooth interior wall tubes.
According to yet another aspect of the present invention there is provided a high pressure transfer device comprising: A housing. A pocketed rotor containing a plurality of through going pockets, the rotor rotatable about a given axis of rotation and the pockets having opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor and the pockets are provided in at least first and second sets (preferably with the pockets in each set offset from the pockets in the at least one other set). A housing enclosing the rotor, the housing having an exterior periphery and first through fourth ports disposed around the exterior periphery thereof for registry with the inlets to and outlets from the through going pockets, for each set; for each set the first port being opposite the third port; and the second port opposite the fourth port. The rotor mounted in the housing for rotation with respect to the ports about the given axis of rotation. And, the pockets filled at side portions thereof with flow precluding inserts.
According to yet another aspect of the present invention there is provided a method of enhancing the flow transfer characteristics of a high pressure transfer device comprising: A high pressure transfer device comprising: a housing; a pocketed rotor containing a plurality of through going pockets, the rotor rotatable about a given axis of rotation and the pockets having opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor and the pockets are provided in at least first and second sets; a housing enclosing the rotor, the housing having an exterior periphery and first through fourth ports disposed around the exterior periphery thereof for registry with the inlets to and outlets from the through going pockets, for each set; for each set the first port being opposite the third port; and the second port opposite the fourth port; the rotor mounted in the housing for rotation with respect to the ports about the given axis of rotation; the method comprising: (a) Substantially filling the nooks, crannies, and related flow restrictions in the pockets to provide a more uniform flow area through the pockets. And, (b) rotating the rotor about its axis while causing a low pressure slurry to flow from the second port through the rotor pockets to the fourth port, and causing a high pressure liquid to flow from the first port through the rotor pockets to the third port.
In the method (a) and (b) may be practiced so that the flow in (b) is more uniform by at least 5% (e.g. more than 10%, or even more than 15%) than if (a) were not practiced. In the practice of the method (a) may be practiced filling the nooks, crannies, and related flow restrictions with a substantially solid metal, or with an initially fluid but hardenable wear resistant material, or with an at least partially hollow insert.
According to yet another aspect of the present invention there is provided a method of constructing and operating a high pressure transfer device comprising a housing having opposite first and third ports, and opposite second and fourth ports and a rotor mounted for rotation within the housing, the method comprising: (a) Providing at least two tubes defining substantially smooth interior wall through extending pockets in the rotor. And, (b) rotating the rotor about its axis while causing a low pressure slurry to flow from the second port through the rotor pockets to the fourth port, and causing a high pressure liquid to flow from the first port through the rotor pockets to the third port.
In the practice of the method described above, (a) may be accomplished by mounting at least two tubes in a substantially cruciform position in the rotor. Further (a) may be practiced using substantially polygon cross-section tubes, or circular or elliptical cross-section tubes.
It is the primary object of the present invention to provide for enhanced operation and flow of a high pressure transfer device. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.